altcoin is a term given to any cryptocurrency other than bitcoin, it stands for alternative coin (alternative cryptocurrency) which is alternative to bitcoin.

which is better?
bitcoin without a doubt. because it is the biggest crypto and has the largest number of usages and investors.

the other two that you named are dead (dying) coins in my opinion and offer no special feature.
litecoin is a copy of bitcoin, and dogecoin is still alive because of its community and because it is a fun coin.